How do I mirror across anything other than X, Y, or Z axis?
Use two mirror modifiers, both one along the X+Y axis.
Once you have the Y and X mirrored once, then you can add an empty and use it as a mirror object to mirror it along the x+y for your second modifier.


Simply position the empty to where you want to objects to be mirrored. The empty will essentially replace the object's origin for the mirror modifier.